For years, clinical ultrasound systems measurement has incorporated either pulsed or continuous wave (CW) ultrasound techniques for imaging of tissue structure and flow of blood therethrough. Since the tissue investigated is a dispersive medium, the signal transmitted into and thereafter reflected from tissue discontinuities suffers significant attenuation. That is, the greater the path taken by the acoustic signal within the subject, the greater the signal is attenuated and otherwise changed. Previous systems have included compensation techniques such as time-controlled gain to provide correction for the anticipated attenuation by the signal in the subject tissue. Similarly, other correction techniques have been applied with varying degrees of success.
Regardless of the correction techniques used thus far, certain problems remain. Typical of these problems are the multiple reflections incurred between the specimen surface to be investigated and the surface and the transducer within the ultrasonic probe. Moreover, for the deeper signal penetration levels, the signal becomes uncorrectably attenuated and unfocussed, often obscuring critical imaging information.
These problems remain, since existing Doppler systems typically assume a signal propagation model incomplete for all applications. As a result, the resolution and/or range of Doppler or pulse-Doppler systems are unnecessarily limited.